Databases 2 Retrieving information. Lecture 19 COMPSCI111/111G SS 2016

Transcription

1 Databases 2 Retrieving information Lecture 19 COMPSCI111/111G SS 2016

2 Today s lecture Recap of yesterday s lecture Using Queries to retrieve information from database Using Reports to retrieve information from a database

3 Recap Databases can use the relational model, where relationships exist between entities Relationships require tables, primary key and foreign key. Referential integrity is an important concept Looked at how to create tables, insert fields and data and create a relationship

4 Aspects of a database Before we can create our database, we need to decide how to: 1. Organize data in our database Models, tables, relationships 2. Enter data in our database Datasheet view 3. Retrieve data from our database 4. Present the retrieved data to the user

5 Retrieving data - queries Queries allow you to retrieve certain records from your database Two kinds of queries in Access: Query by example (QBE): Visual way of designing queries Access converts your QBE queries into SQL SQL (Structured Query Language): Uses commands to retrieve data from databases Developed by IBM in the late 1970 s Access creates a table containing the results of the query

6 QBE queries

7 QBE queries QBE grid Choosing fields Adding criteria to the field

8 QBE queries Run button Query results

9 QBE queries - sorting Results from QBE queries can be sorted in ascending and descending order

10 QBE queries - expressions Fields can be combined together to create an expression with the Expression Builder [reserves]![num_reptiles]+ [reserves]![num_amphibians]

11 QBE queries A Totals QBE query allows us to group data using functions such as Min, Max, Avg, Sum etc. Totals button

12 QBE queries

13 QBE Exercise Complete this QBE grid so that it will return the first names, surname and grade (in that order) of all students who have received an A+. Sort the results by surname in alphabetical order

14 QBE Exercise

15 SQL introduction Structured Query Language (SQL) was developed by IBM in the 1970s and is commonly used today It uses text commands to perform operations on databases, such as inserting and removing records and running queries

16 QBE queries

17 SQL queries Four clauses that can be part of a simple SQL query: SELECT FROM WHERE ORDER BY Construct a SQL query that will return the first names, surname, and grade (in that order) of all students who have received an A+. Sort the results by surname in alphabetical order

18 SQL queries - SELECT Selects fields from the tables that we want to display in our results table Syntax: SELECT [comma separated list] SELECT [First Names], Surname, Grade Note the square brackets around First Names needed because of the space

19 SQL queries - FROM Specifies the table which holds the field(s) listed in the SELECT clause Syntax FROM [comma separated list] SELECT [First Names], Surname, Grade FROM Students

20 SQL queries - WHERE Optional; used to provide criteria that limit the records displayed in the results table Syntax WHERE [criteria], [criteria], There are a range of criteria we can use: Comparisons (=, >, <, <=, >=, <>) e.g., WHERE [Land Area] < BETWEEN AND e.g., WHERE Price BETWEEN 10 AND 20 LIKE (some pattern) e.g., WHERE [City] LIKE San *' AND, NOT, OR (combined with any of above) e.g., WHERE Country = New Zealand' AND City = Auckland' IS NULL, IS NOT NULL e.g., WHERE [Postal Code] IS NOT NULL

21 SQL queries - WHERE SELECT [First Names], Surname, Grade FROM Students WHERE Grade = A+

22 SQL queries ORDER BY Optional; allows us to sort our data in ascending or descending order Syntax: ORDER BY [name of field] [ASC/DESC] SELECT [First Names], Surname, Grade FROM Students WHERE Grade = A+ ORDER BY Surname ASC

23 SQL queries You need to ensure that you put a semi-colon on the last clause of your SQL query: SELECT [First Names], Surname, Grade FROM Students WHERE Grade = A+ ORDER BY Surname ASC;

24 SQL queries We run a SQL query in the same way that we run a QBE query Run button

25 SQL exercise Write an SQL command that will only display the first name, surname and grade of students whose Total mark was greater than 70. Order the results table by ID number in ascending order

26 SQL exercise SELECT [First Names], Surname, Grade FROM Students WHERE Total > 70 ORDER BY ID ASC;

27 Aspects of a database Before we can create our database, we need to decide how to: 1. Organize data in our database Models, tables, relationships 2. Enter data in our database Datasheet view 3. Retrieve data from our database QBE and SQL queries 4. Present the retrieved data to the user

28 Reports Reports allow you to present the contents of a table, query etc. in a nicely formatted table There are two ways of creating Reports: Report Tool (show entire table, some formatting control) Report Wizard (table/field selection, grouping, sorting)

29 The Report Wizard Select the tables and fields you want to display in your report

30 The Report Wizard You can group records in the report using particular fields

31 The Report Wizard You can sort records in the report by one or more fields

32 The Report Wizard You can set certain aspects of your report s formatting in the Wizard The final step involves giving the report a name and clicking on Finish

33 The Report Wizard The finished report, ready for printing You can continue to modify the report s formatting at this point

34 Summary 1. Organize data in our database Models, tables, relationships 2. Enter data in our database Datasheet view 3. Retrieve data from our database QBE and SQL queries 4. Present the retrieved data to the user Report Wizard